Optical fibers are constructed of a cylindrical core of a first material, typically ceramic such as silica, or glass, surrounded by a cladding of a second material that can be similar to that of the core. A protective jacket can be provided about the cladding, formed typically of a polymer. The core material of an optical fiber has a higher index of refraction than its cladding, therefore optical signals can be made to propagate along the core and are totally internally reflected.
In 1978 it was discovered that, in optical fibers in which the core material could be made to undergo a change in refractive index upon exposure to light of a particular wavelength at a threshold intensity, diffraction gratings could be written by optically writing into the fibers alternating regions of relatively higher and relatively lower refractive indices (see, for example, Hill, et al. Appl. Phys. Lett. 32: 647-49, 1978). Since their discovery, in-fiber gratings have found application as temperature and strain sensors, fiber optic mirrors, filters, mode converters, wavelength demultiplexers, etc. These and other applications define, in part, a need for the development of reliable and economic method for fabricating in-fiber gratings.
An "internal writing technique" for optically writing a diffraction grating in an optical fiber involves introducing light into an end of an optical fiber and allowing an interference pattern to be established with counter-propagating light (Fresnel-reflected from the far end of the fiber) to form a standing wave in the fiber. In a fiber in which the core is photosensitive (able to undergo a change in refractive index upon exposure to radiation), the refractive index of the fiber core is altered disproportionately greatly at points of constructive interference, thus a refractive index perturbation (index grating) is formed that has the same spatial periodicity as the interference pattern. Such a refractive index grating acts as a distributed (Bragg) reflector (see Hill et al., "Photosensitivity in Optical Fibers", Annual Review of Materials Science, 23 125 (1993); Vengsarkar, et al., "Long-period Fiber Gratings as Band-Rejection Filters", Journal of Lightwave Technology, 14:1 (January, 1996).
A later-developed "external writing technique" for creating in-fiber optic gratings involves irradiating an optical fiber with two separately-oriented laser beams originating from a single beam which is split and then made to intersect at the optical fiber core. The intersecting beams form an interference pattern within the core of the fiber and, if the fiber core is photosensitive, regions at which the beams constructively interfere undergo a disproportionately high change in refractive index, thus an index grating is written in the fiber core (See Meltz, et al. "Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method", Optics Letters, 14, 823 (1989)). With the external writing technique, an additional degree of freedom for writing refractive index gratings exists since the period of the interference pattern depends not only on the wavelength of light used for writing (the only adjustable parameter in the internal writing technique) but the angle between the two interfering laser beams affects the period of the interference pattern within the optical fiber core. With external writing, gratings can be written in standard telecommunications fibers to be Bragg-resonant at wavelengths of interest for fiber optic communications. (See Kashyap et al., Electron. Lett., 26 730-32 (1990)).
Another technique for external writing of gratings in optical fibers is a "point-by-point" technique. This involves translation of an optical fiber, by precision motors, past an aperture through which light from a light source passes and strikes the fiber. A portion of the fiber is positioned adjacent the aperture and exposed to the light source, exposure is discontinued, the fiber is moved to expose a different portion to the aperture, and the process repeated until a particular set of regions of the optical core have been exposed to light inducing a change in refractive index of those regions. (See Hill, et al. Electron. Lett. 26, 1270-72 (1990); and Malo, et al., "Point-by-Point Fabrication of Micro-Bragg Gratings in Photosensitive Fibre Using Single Excimer Pulse Refractive Index Modification Techniques" Electron. Lett., 29 1668-69 (1993)).
Another external writing technique involves passing light through a grating phase mask or amplitude mask placed near or adjacent an optical fiber having a photosensitive core and illuminating the fiber core through the phase mask. A diffraction pattern is thereby generated and applied to the fiber core, thus writing a grating in the core as described above (See Hill, et al., "Bragg Gratings Fabricated in Monomode Photosensitive Optical Fibers by UV Exposure Through a Phase Mask", Applied Physics Letters, 62, 1035 (1993)).
Askins, et al., "Fiber Bragg Reflectors Prepared by a Single Excimer Pulse", Opt. Lett., 17, 833, 835 (1992), and Dong et al., "Single Pulse Bragg Gratings Written During Fibre Drawing", Electron Lett., 29 1577-1578 (1993) describe exposure of a photosensitive optical fiber to a single 20 nanosecond excimer laser pulse to write a grating in the fiber core. This technique is known as the "single shot" method, and requires a very powerful radiation source.
While the above-described techniques find use in some circumstances, the internal writing technique does not allow freedom in variation of the period of the interference pattern created sufficient to produce gratings for several applications nor, typically, can it be written without a DC component from the source undesirably affecting the grating. As for the external writing techniques, the interference, phase mask, and point-by-point techniques often are complicated by instability between the light source, optical fiber and mask or aperture. If all components are not completely stable with respect to one another, inaccuracy and imprecision in the resultant grating can occur. Additionally, the interference, single-shot and point-by-point techniques involve relatively complicated and expensive apparatus.
Additionally, techniques such as the internal writing, external writing interference, and external phase mask techniques typically result in a pattern that is a sinusoidal variation in index of refraction, rather than a square wave index pattern or patterns with more complex geometries.
Another field that involves fabrication in connection with small-scale articles such as cylindrical articles is the field of microelectronics. Miniaturization of electrical components has created a need for microinductors and microtransformers. The large number of turns per unit length required for these structures makes fabrication of miniaturized inductors and transformers challenging. While several different methods for the generation of planar microtransformers using conventional silicon processing techniques have been developed, conventional techniques such as photolithographic processes can form high resolution patterns on planar substrates but lack the depth of focus to pattern non-planar substrates. Three-dimensional structures typically are obtainable only through stepwise addition or removal of planes or strips of material, a labor and material-intensive process, that limits the possible geometries.
Kawahito, et al., in an article entitled "A Fluxgate Magnetic Sensor With Micro-Solenoids And Electroplated Permalloy Cores", Sensors and Actuators A, 43, 128-134 (1994), describe a fluxgate magnetic sensor produced via silicon micro-technology. For a rod-core sensor, a relatively complicated fabrication process is involved including groove formation, round etching and oxidation, electron-beam lithography, evaporation and liftoff, SiO.sub.2 sputtering and Cu electron-beam evaporation, Cu patterning and resist patterning, NiFe selective electroplating, Cu film removal and planarization, and through-hole and Al patterning.
In an article entitled "High-Resolution Microcoil .sup.1 H-NMR For Mass-Limited Nanoliter-Volume Samples", Science, 270, 1967-1970 (Dec. 22, 1995), Olson, et al., described a polyimide-coated fused-silica capillary surrounded by a microcoil for use in proton micro-NMR spectroscopy. The microcoil was made from 50 micron diameter wire, and was wound about the cylindrical substrate. Adhesive was applied to adhere the coil to the substrate. Coil fabrication was monitored with a dissecting stereomicroscope.
While fabrication of three-dimensional microelectronic devices has yielded some successes, known techniques for such fabrication are relatively complicated.
Stents are tiny scaffolds that can be introduced into a blood vessel, typically a coronary artery after balloon angioplasty, and expanded in the vessel to hold the vessel open. Because stents are cylindrical structures of metal with feature sizes in the range of 50-100 microns, they can be difficult to produce using conventional methods and, as a result, costs are high. U.S. Pat. No. 4,655,771 (Wallsten) describes a stent for transluminal implantation. The device can be fabricated by weaving, and points at which filaments defining the stent cross can be welded together.
Accordingly, it is an object of the present invention to provide a simple, relatively inexpensive technique for optically writing refractive index patterns in photosensitive articles that is adaptable for a wide variety of end products, that utilizes simple, readily-available materials, and that eliminates instability problems.
It is another object of the invention to provide a simple, relatively inexpensive technique for fabricating small-scale, three-dimensional, preferably cylindrical metal structures such as microtransformers, microinductors, and stents.